spapr: Eliminate SpaprIrq:get_nodename method

spapr: Eliminate SpaprIrq:get_nodename method

This method is used to determine the name of the irq backend's node in the
device tree, so that we can find its phandle (after SLOF may have modified
it from the phandle we initially gave it).

But, in the two cases the only difference between the node name is the
presence of a unit address.  Searching for a node name without considering
unit address is standard practice for the device tree, and
fdt_subnode_offset() will do exactly that, making this method unecessary.

While we're there, remove the XICS_NODENAME define.  The name
"interrupt-controller" is required by PAPR (and IEEE1275), and a bunch of
places assume it already.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Greg Kurz <groug@kaod.org>